1. Field of the Invention
This invention relates to scrambler speech transmission systems, and, in particular, to means for scrambler speech synchronization over narrow-band (e.g., telephone or radiotelephone) channels to make the transmission secure against accidental or intentional eaves dropping. Accordingly, it is a general object of this invention to provide new and improved systems of such character.
It is a further object of the invention to accomplish scrambler speech transmission and synchronization with the aid of one or more "keys" (i.e., physical keys, key numbers, key words, or key groups of alphanumeric or other characters or symbols) prearranged between or among the communicating parties, which keys can be quickly and easily changed.
2. Description of the Prior Art
There are a wide variety of old systems for speech scrambling. For purposes of comparison with the instant invention, three old systems are discussed.
The first two systems yield scrambled speech with the same bandwidth as the original speech, but they do not utilize a key. The first system, familiar to those who have played a phonograph record backwards, is time inversion scrambling. The speech is first recorded, then the recording is played in reverse time sequence. The second system is the frequency counter-part of the first system, namely frequency inversion scrambling. In this system, the frequency spectrum of the original speech is inverted, e.g., by the process of single sideband modulation onto the lower sideband of a carrier.
The third old system is a generic form of key encoded scrambling called time sequence scrambling. In this system, short intervals of the original speech are selected for transmission in a permuted order. The order in which the segments are selected is determined by the prearranged key. Time sequence scrambling can be accomplished with either the original analog speech signal or with a digitally-encoded version of the signal (in which case digital pulses are scrambled).
All of the above systems for scrambling suggest the corresponding systems for unscrambling. For time inversion scrambling, the process is repeated, thereby restoring the signal to its original time sequence. For frequency inversion scrambling, repetition of the process restores the spectrum to its original condition. For time sequence scrambling, the segments are selected from the incoming signal by means of an "unscrambling key." This key is the permutation of consecutive integers which, when used to select the integers of the "scrambling key", yields a set of consecutive integers.
Non-key-encoded scramblers, having only one mode of scrambling per device, are useful in commercial telephone service for providing a modicum of security in business communications, particularly where accidental cross-connection might occur to a third party. However, the more widely these devices are used, the less security they offer. A key-encoded scrambler is secure, even in proliferated use, because different users can use different keys.
The probability of a transmission being unscrambled by a third party can be kept very low, even if the third party has the unscrambling apparatus, provided that the number of different keys is very large.
An application for a key encoded scrambler is in a police radio. It would not do to use a non-key-encoded scrambler, because those criminals who use police radio receivers to monitor the reactions of the police would quickly acquire unscrambling apparatus, particularly if simple inversion techniques were used. With key encoding, the security is in the code (which can easily and frequently be changed), not in the apparatus. Thus, key-encoded scrambling systems can be used in a wide variety of applications: in police, military, civilian, and government communications, and in industry, in a wide variety of different confidential or secret business communications affecting finances, trade secrets, account data, personnel data, etc.
A principal problem with key encoded scrambling systems utilizing time sequence scrambling is that they cannot be used to obtain an unscrambled signal restored to the form of the original signal unless the signal is sent over a wideband channel, thus precluding the use of ordinary telephone or radiotelephone channels. When analog sequences are scrambled, the amplitude of the scrambled signal has abrupt discontinuities at the junctions of successive segments. To avoid distortion in the unscrambled signal, the discontinuities are preserved in transmission, which requires that the signal be sent on a wideband channel. When digital sequences are scrambled, the process of analog to digital conversion yields a wideband signal, which also is preserved in transmission through the use of a wideband channel.
Another principal problem with key encoded scrambling systems, to which this invention is particularly addressed, is the problem of synchronization: that is, the provision for signals to be sent and received over the same narrow-band or wide-band channel in such a manner as to control automatically the decoding at the receiver in the exact time sequence, determining just when the sequence is to start.
3. Prior Art Statement
As stated above, there are a wide variety of old systems for speech scrambling. A brief discussion of the prior art is set forth at col. 1, lines 20-45 of U.S. Pat. No. 3,225,142 to Schroeder, reproduced below:
"Analog communication systems have been devised in which electric signals corresponding to speech or other signals to be privately transmitted have been scrambled, garbled, or otherwise rendered more or less unintelligible in various ways. Their frequency components have been inverted with respect to a selected nominal frequency; they have been broken up into short segments which are then transmitted in alternation with corresponding short segments of another message; they have been recorded, inverted with respect to time (picked up backward) and transmitted so inverted; their transmission rate has been widely varied at a fairly high rate; and many other devices have been employed to make the relation between the electric signal as transmitted and the original message to be transmitted unobvious and therefore undecipherable. Such systems are characterized, in the main, by a perturbation or other masking effect which follows a definite and prescribed pattern, either recurring periodically in time or involving a definite numerical relation between the component frequencies of the plain message and those of the ciphered message. In general, this pattern is separate and distinct from the message to be transmitted and is unrelated to the peculiarities of the particular message, being controlled entirely by means external to portions of the apparatus or circuit which carry the message signals themselves."
The foregoing reference includes, in the opinion of the applicants and their attorney, the closest prior art of which they are aware. This statement shall not be construed as a representation that a search has been made or that no better art exists.
The relevance of the cited reference is in the paragraph quoted above, as a statement of prior art which exists.